1. Field of the Invention
The present invention relates in general to an ink-jet printing apparatus, and more particularly to an ink jet printing apparatus which is improved in its flushing operation to recover the desired quality of printing.
2. Discussion of Related Art
An ink jet printer is known as an example of an ink jet printing apparatus arranged to eject droplets of an ink onto a recording medium such as a sheet of paper, for printing an image of the recording medium. In this ink jet printer, a head unit having at least one ink jet head is provided with an ink cartridge which accommodates the ink and which is installed on each ink-jet head such that the used ink cartridge is replaceable with a new one. In operation of the head unit, the ink is supplied from the ink cartridge to each ink jet head having ejection nozzles, so that a printing operation is performed by ejection of the ink droplets from selected ones of the ejection nozzles.
The ink jet printer of the type described above suffers from a problem that air bubbles or foreign materials remaining in ink flow passages disturb ejection of the ink from the nozzles, resulting in deterioration of the quality of printing. To solve this problem, there has been practiced a so-called “purging operation” wherein the ink is forcibly discharged from end portions of the ejection nozzles which are open in a nozzle surface of each ink jet head. This purging operation is implemented, upon initial ink supply to the ink jet head or replacement of the ink cartridge, or at a suitable time during use of the ink cartridge, by a manual operation of a switch by the user of the ink jet printer, or automatically when a predetermined condition for implementing the purging operation is satisfied. The purging operation is performed by pressure-tightly covering the nozzle surface with a suction cap, and applying a negative pressure to the interior space of the suction cap by operation of a suction pump, so that the ink is positively discharged by suction out of the ejection nozzles of the ink jet head through the suction cap.
The purging operation is followed by a wiping operation to wipe the nozzle surface, and a flushing operation in which the ink jet head is operated by application of a drive signal, to eject the ink droplets from the ejection nozzles, as in a normal printing operation of the ink jet printer. The flushing operation permits removal of the residual ink or foreign materials from the open end portions of the ejection nozzles if the residual ink or foreign materials have been forced into the open end portions during the wiping operation, and removal of air bubbles or foreign materials from the ink jet head if the air bubbles or foreign materials have been drawn into the ink jet head. The flushing operation is also effective to recover uniformity of meniscus of the ink in the open end portions of the ejection nozzles.
The ink cartridge used for supplying the ink to the ink jet head of the ink jet printer arranged as described above generally has an outer casing formed of a suitable material such as polypropylene, and a porous member which is disposed within the outer casing and which is impregnated with the ink. The interior of the outer casing is deaerated or degasified when the ink cartridge is shipped. When the ink cartridge is installed on the ink jet printer, the ink within the ink cartridge is exposed to the atmosphere through an opening of the ink cartridge, so that the air is gradually dissolved in the ink, and the ink is eventually saturated with the air.
During a purging operation on the ink jet head, small air bubbles are produced within the suction cap, as a result of a high rate of flows of the ink by suction. The ink containing the air bubbles are drawn from the ejection nozzles into the ink jet head, in the presence of a back pressure generated within the ink cartridge, due to a capillary force of the porous member. When the ink within the ink cartridge is highly deaerated, the air bubbles disappear as a result of rapid dissolution of the air bubbles in the ink. When the ink is saturated with the air as a result of gradual reduction of the deaeration, it takes a long time for the air bubbles to be totally dissolved in the ink. If a flushing operation is performed on the ink jet head immediately after a purging operation of the ink jet head, the flushing operation permits only a portion of the air bubbles to be discharged from the ink jet head, and tends to promote a growth of the residual air bubbles due to a pressure variation within the ink cartridge as a result of the flushing operation.
To solve the problem described just above, it has been proposed to estimate the time required for totally or considerably removing the air bubbles produced by the purging operation, and initiate the following flushing operation only after the estimated time has passed after the termination of the purging operation. An example of this solution is disclosed in U.S. Pat. Nos. 6,036,299A and 6,305,778B1.
Where the purging operation is performed using the same suction cap for a plurality of ink jet heads corresponding to respective different colors, flushing operations must be performed a considerably large number of times after the purging operation, in order to prevent mixing of the inks of different colors. In this case, the residual air bubbles tend to grow during the repeated flushing operations, even when the flushing operations are initiated the predetermined time after the purging operation.